Proper (professional) way to texture paint?
Texture Painting is as varied in style as the actual mesh modeling and sculpting that you do as well - not everything is made with PBR in mind, so that is why you see tutorials that cover what would be considered to be 'handpainted' style texturing for low poly games.

When painting with realism in mind, I utilize an AO texture bake to start with for multiple uses as in masks for wear,dirt, grime, scratches, etc. I use a color map for the main colors of the material, and then I paint roughness and metalness maps for plugging into the shader. I use custom brush images for patterns, and I play around a lot with setting up bump from the different stencils so that I get a decent amount of realism - but even through all this, I have focused mainly on texture paint with faking realism and less on baking down the output to get it ready for games, as I usually only go so far as to get a still image ready for a packaging demo or a product shot for a job at work - otherwise, I just create what I want for my own taste.

If you haven't already done so, check out BPainter add-on for 2.79 as it gives you a lot of really good advantages when painting in Blender - lots that help with organizing nodes in the background and such.